The invention relates to the replacement of missing video in a video signal and, more particularly to a digital dropout compensator with multiple color television standard capability, which selectively replaces missing video data with averaged data from a selected line or lines, or with recirculated data. The replacement data is selected in response to an adaptive decision generated by the location and characteristics of the missing video data.
As known in the art, defects in a television signal produce objectionable picture characteristics when the signal is reproduced as on a television screen. In broadcast television signals these defects arise from RF loss and impose noise on the signals which readily are visible in the television picture. These defects which arise from noise or loss of video RF are referred to in the art as "dropouts".
Since recorded television images corresponding to the television signal contain redundancy from line to line, it is possible to substitute segments or entire lines of the data from adjacent lines in place of the defective segments. To this end, early generations of dropout compensators (DOC) store image information from two lines previous and, when a dropout occurs, insert the previous line of information, or a portion thereof, into the video signal in place of the missing video. However, the process of replacing dropouts by previous line data or by any data is complicated by the characteristics of the signals in composite color television. As well known, in an NTSC color television standard, color television signals include a wideband luminance signal and a phase/amplitude modulated color subcarrier. The latter however, occurs with a 180.degree. phase difference from line to line. This phase difference precludes direct signal substitution between adjacent lines when correcting dropouts because the substituted color information would be incorrectly reproduced. Since the above DOC derives the replacement video from two lines previous, the chroma is in proper phase. However, such video spatially is sufficiently distant from the dropout to have less redundancy whereby the replacement video is a less accurate estimate of the dropout-affected video.
A subsequent generation of DOC's attacked the color phase reversal problem while improving dropout replacement, by substituting a signal which is derived from multiple data samples of a previously stored line. The replacement data is derived by averaging the signal from two data points displaced from the dropout point by approximately .+-.1/2 of the color subcarrier period. This provides a signal wherein the color subcarrier is in phase with an adjacent line, and the luminance signal approximates the average of the luminance signal over a color subcarrier period.
Still other DOC systems have been developed which produce a similar result by generating a replacement signal which is the average of four data points, two taken from a following image line and two from a previous image line relative to the image line currently being displayed. This latter DOC system of averaging four data points from adjacent lines, generally employs two serially connected 1H delay lines in combination with selected adders to provide the required delays and summations for performing the desired averaging of the four data points. In such a system, the signals first are variously delayed by one-half the color subcarrier period and/or by one color subcarrier period, as generally known in the art, to provide the desired data alignment prior to then summing the results. The summation is then divided by four to produce the replacement signal equal to the average of the four data points. Although this scheme of deriving replacement data from surrounding data points provides more accurate data, it cannot handle extended dropouts of greater than one line. In addition, the scheme requires added circuitry and cannot be adapted to multiple color television standards but has been used primarily either with the NTSC or the PAL standard.
An extension of the above system of averaging multiple surrounding data points employs a complex delay line system which is selectively tapped to provide the desired delays and subsequent additions to perform the averaging function. Such system employs a common delay element whereby all of the tapped signals undergo similar signal processing. In this latter averaging scheme utilizing the common delay element, the summed signal from the succeeding horizontal line may be attenuated with respect to the summed signal from the previous horizontal line, but the attenuation generally is compensated by including gain in the second adder circuit, thus simplifying the circuitry.
The two systems for averaging surrounding data points relative to a dropout in a middle line of a television signal, provide a more accurate replacement signal when compared to the system which merely replaces the dropout with corresponding information from the previous one or two lines. However, the averaging scheme generates noticeable distortion or blurring in pictures having sharp horizontal or angular edges, that is, where there is an intensity change in the vertical direction. To overcome the loss of resolution in the vertical direction, still another averaging scheme selectively records the video such that the originally adjacent samples are not adjacently recorded on the recording media. During reproduction, if a dropout occurs, the difference between samples in one direction with respect to the dropout sample is computed, and the difference between samples in at least one other direction with respect to the dropout sample also is computed. The signal with the least difference is used to control the direction of data from which the average is computed, which then is used as the replacement data for the dropout sample.
While the various averaging schemes improve the accuracy of replacement data, they are only useful in situations where the dropout is shorter in length then one line of the recorded signal. Such schemes cannot handle extended dropouts of greater than one line duration, which does occur in the field of data recording and reproducing. In addition, since such schemes provide the 1H delay lines prior to the function of chrominance signal inversion to compensate for the differences in chroma phase from line to line, such schemes have been utilized with signals in a single color television standard. That is, such averaging schemes are not readily adaptable to a switchable implementation capable of readily accommodating NTSC, PAL, and SECAM color television standards.
Nonetheless, it is highly desirable in this era of sophisticated, high speed, color television recorders, to provide an associated time base corrector device wherein the dropout compensator provides not only the most highly accurate replacement data for a dropout, but which is also readily switchable between the NTSC, PAL, and SECAM color television standards. To date there are no systems available which are fully digital, and can provide dropout compensation for multi-standard signal systems, while further providing the best possible replacement data for a particular dropout particularly in systems where data is being recorded at non-standard speeds.
The present invention provides a fully digital adaptive dropout compensator (DOC) which generates optimum dropout replacement data, and which further is capable of supplying the replacement data for all of the various color television standards, due to the selective arrangement of components forming the dropout compensator. The system is simpler in implementation than the dropout compensators of previous discussion, and provides more accurate replacement data by tailoring the replacement data source to the type of dropout. To this end, characteristics such as the extent of the dropout and the closest good data available, are detected and an adaptive decision is made, based on the characteristics, to supply replacement data from the most optimum adjacent data available. That is, the invention contemplates replacing dropouts from any of the various available data in selected adjacent lines in response to the adaptive decision. For example, if good video is available in adjacent lines the present adaptive DOC replaces a dropout either by averaging data from the previous line of data, from the following line of data, or by spatially averaging data from both the previous and the following adjacent lines. If video data from multiple adjacent lines is missing, the adaptive logic generates the decision to supply recirculated data from a previous line or lines as the replacement data. Thus the invention provides a multi-standard dropout compensator with multi-standard adaptive logic for detecting which available data source provides the most accurate replacement data for the specific dropout, and which then selects such preferred data to replace the dropout.
The advantageous accuracy, versatility and simplicity of the present adaptive DOC are derived through its improved configuration. To this end, in the absence of a dropout, a first input switch routes the video data through a small delay equivalent to the delay caused by a chroma inverter also in the DOC, and thence through a one-line delay, to derive the output of the dropout compensator. More particularly, the chroma inverter is located at the front of the present adaptive dropout compensator immediately behind the video input switch, with the one line delays located towards the end of the converter downstream of the chroma inverter. This allows the strategic insertion of various digital adders, standard switchers and an adaptive switch, as further described, which allows the use of the same digital circuit paths and thus the same digital circuit components in all standards of operation.
In the presence of a dropout, the adaptive logic of previous mention validates that the previous line and the following line of data are good, whereby the adaptive logic then selects a corresponding position of an adaptive output switch to provide replacement data derived by spatially averaging data from the previous and following lines. If the line previous or the line following have defective or missing data, the adaptive logic selects an adaptive output switch position which provides averaged replacement data from the other good line of data. If all three adjacent lines of data contain dropouts, then the adaptive logic selects a switch position which provides recirculated data from a previous line or lines to replace the dropout on the line currently being displayed.
The system further includes the standards switches of previous mention within the chroma inverter, within a recirculate data loop, and prior to the adaptive output switch, which switches cooperate within the adaptive DOC to provide switchability between the NTSC, PAL and SECAM color television standards in accordance with the invention combination.